Magic face cloths - the ones that expand when you wet them - supermarkets do them. Bath bombs, toothbrush with favourite character on, notepad/mini colouring in book, Playmobil/similar figure in those foil bag things - you can sometimes pick them up for £1 if they're an old series, a packet of raisins, crayons, bath squirty toy, whoopee cushion.

Wilkos do some cute novelty stationary items from 35p-£1 each - things like penguin rubbers, mouse pens, shark pencil sharpeners, heart shaped post-it's, etc etc. dd has adored this range since she was about 2.Other ideas: plasticine; sheets of stickers; those mini craft sits you can get for about £1 in supermarkets, Wilkos etc.